This invention relates to a solar battery, and particularly to a solar battery which is composed of a semiconductor having "p-n" junctions provided in the form of multi-lamination.
As a result of the recent energy shortage, an energy source other than petroleum has been sought, and interest has increased in the use of solar energy, which is available in infinitely great amounts and is non polluting. As a means for converting solar energy to electric energy, a solar battery was invented in 1954 and the battery has been continuously studied from the practical point of view. However, the practical use of the solar battery has been limited only to artificial satellites or unmanned lighthouses. This relatively narrow application of the solar battery is attributable to its high cost which makes it non-competitive with commercially available electric power. Therefore, the enlargement of application of solar batteries will be impossible unless the production cost of semiconductor materials can be reduced.
And at the same time, it will be necessary to raise the solar-to-electric energy conversion efficiency, to extend the life, and to increase the reliability of solar batteries before they can attain practical usage in a wide range of industries.
Increasing the electric power obtainable from a solar battery is of primary importance. The generation rate G of hole-electron pairs in a semiconductor when exposed to a monochromatic light, is given by the following equation: EQU G(x) = .alpha.(.lambda.).PHI..sub.0 (.lambda.)exp (-.alpha.x) (1)
where .alpha.(.lambda.) is the absorption coefficient [cm.sup.-1 ];.PHI..sub.0 (.lambda.) is the injection light flux density [cm.sup.-2 S.sup.-1 ] and "x" is the distance measured from the surface of the semiconductor body on which the monochromatic light falls.
Only those electron-hole pairs which appear within a diffusion length from the junction region are available for producing electric power. As a result of absorption of the light penetrating into the superficical depth of a semiconductor body, electron-hole pairs appear, but a part of those electron-hole pairs will disappear at recombination centers in the surface of the semiconductor body, and those electron-hole pairs which appear in a relatively deep part of the semiconductor body will be recombined, and will disappear before they reach the "p-n" junction. In view of this, the conventional solar battery having a single "p-n" junction lying in the traverse direction to the incident light, is structurally disadvantageous, and is less efficient in the conversion of the solar energy to electric energy.
The output voltage of a unijunction solar battery depends on the Fermi levels of the "N" and "P" type semiconductor, which compose the "P-N" junction and therefore the use of a semiconductor of high impurity concentration is effective in raising the output voltage of the solar battery. In this case, however, the diffusion length of the excess minority carriers is disadvantageously shortened, and accordingly the efficiency is lowered. In the hope of solving this problem a series-connection of unijunction semiconductor elements and a pile of "p-n" junction semiconductor elements have been hitherto proposed. The pile of "p-n" junction elements are so constructed as to expose the multi-layer side thereof to the light. As a modification thereof, a plurality of "p-n" elements of decreasing dimensions are piled up so as to constitute a plateau.
As seen from the above, the solar batteries hitherto proposed have a complicated structure, and they work at a low efficiency.
The object of this invention is to provide a solar battery of homoepitaxial or heteroepitaxial multi-layered structure of semiconductors which is easy to produce and works at an increased efficiency of solar-to-electric energy conversion.